Actually its only instantly lethal, no save for an Str 12 (or less) if the attack is a crit and a 6 is rolled. But even still, your 9th level guy alone should have been able to do something other than just stand there and die. What was his AC? He should have had at least a +2-3 bonus to armor, and maybe the same for a shield, so his touch AC could have been anywhere from 14+, more if rings of protection or bracers or something were involved.
What about your characters? To have a 9th cohort, you had to be at least 11th level, which means you alone should have been able to wipe out the shadows before they got your cohort dead. Your fighter would get at least 3-5 attacks/round, maybe more, and one each should be enough for those guys. What were you guys doing while your cohort was dying?
Maybe the DM was going overboard throwing just one encounter too many at you, but CR 3 or not, those shadows had only 3hd, avg of what, 23? 25hp?. Any 11th + fighter shouldn't have a problem with two of those.
Scrolls of (Lesser) Restoration, potions of bull's strength, any of these could have temporarily offset the drain long enough to finish the shadows off. Hell, could have run away between you and your other party members forcing the shadows to take (multiple) AoOs from your party in order to get to the cohort. If you guys were out against wraiths, that means you should have had ghost touch weapons, so it shouldn't have been a big deal to take them out.
The point is, their Str drain is potent, but not unfair considering the plethora of options to use against such weak (low hp) creatures that a single Turn attempt could destroy no matter how bad your turn checks were at your levels. If 2 or more 11th+ characters couldn't stop 2 shadows, you almost deserve to lose your cohort.